Abstract:
Tiritiri Matangi is an offshore island which lies 25 km north of Auckland City in the Hauraki Gulf, New Zealand. Consequent to many centuries of Maori and European occupation and farming, most of its forest cover was removed. In 1971, farming ceased and in 1984 an active restoration program commenced. Conservation efforts on New Zealand’s offshore islands are relatively new, thus not many studies have proceeded long enough to efficiently evaluate responses to disturbances or to detect changes in ecosystem structure and function. This study explores the changes in seedling establishment and survival through a 30-year period with aims to investigate whether seedling establishment will respond to active management, an aspect not yet investigated on the island. This long-term study allows for observation of seedling response to the cessation of farming and the initiation of the Tiritiri Matangi Working Plan, which involved extensive planting of native trees, removal of the Polynesian Rat or kiore (Rattus exulans Peale 1848), and the translocation of native biota. In 1981, Neil Mitchell of the University of Auckland set up 24 one meter-square plots in ‘Bush One’, the healthiest remnant forest on the island. Seedlings in the 24 plots were surveyed 15 times over the 30-year period. The first part of the study was conducted by Mitchell from 1981- 2000. Seedlings were tagged with unique numbers in each plot to mark their initial establishment, measured for height and identified to species. In the years prior to the 1993 kiore eradication, Mitchell identified seedlings which had been browsed by kiore, measured them for height and identified them to species. Canopy composition data was gathered throughout the study. The 2011 survey involved gathering and compiling Mitchell’s data for analysis with the addition of 2011 seedling data (seedling height, species); recording leaf litter cover and depth in the plots; analysing the compiled data in a number of ways with excel to calculate results. Temperature and precipitation data for Tiritiri Matangi Island during the course of study was analysed and compared to aspects of seedling regeneration. An overall increase in seedling numbers from 1981 until 2011 was consequent to the removal of livestock and the development of a restoration programme, indicating that prior to restoration efforts, there were mechanisms which limited successful recruitment and establishment. Although there was an overall increase in seedling numbers, not all species behaved the same way and differences between plots were apparent. Damage to seedlings by kiore was observed, however seedling persistence after damage in addition to the absence of a relationship between eradication and seedling growth or establishment of seedlings meant that kiore were not detrimental to seedling survival. It was instead found that kiore were limiting recruitment through the selective browsing of seeds, acting as a filter for the flora community of Bush One. Kiore were found to have an impact on the patterns of recruitment and have likely had an impact on the successional pathways of Bush One. Bird dispersed species were observed growing in the plots before the start of the restoration programme and there was no great increase by 2011. Despite dispersal of seed into the forest, there was little increase in species richness and a turnover of species throughout the study was revealed; many seedlings did not successfully establish; all species were characterised by low mean growth rates and persistence times; thus limitations to successful seedling establishment and survival in Bush One was [i.e. were] evident. Mechanisms found in this study to have no effect on seedling establishment and survival were the increased leaf litter as observed in 2011, environmental factors, and the introduction of native birds. Kiore were found to have the greatest impact on seedling recruitment and establishment as a result of their seed predation on native species. Increased soil health after the cessation of farming likely aided in successful seedling establishment and survival, but further study is needed to confirm this. With aims to identify the effects of the restoration programme on seedling establishment and survival on Tiritiri Matangi Island, a long term study such as this is essential for observations of the mechanisms that limit or promote successful establishment. Findings may aid in management of all offshore islands with similar histories of occupation.